Fix an issue with editing pre-space objects using a form with no visibility controls

Summary:
WMF ran into this after their update. Here's the setup:

• When you enable Spaces, we leave all existing objects set to null, which means "these belong to the default space". This is so we don't have to go update a trillion objects.
• New objects get set to the default space explicitly (PHID-SPCE-...) but older ones stay with null.
• If you edit an older object (like a task) from the time before Spaces, and the form doesn't have a Visbility/Spaces control, we would incorrectly poplate the value with null when the effective value should be the default space PHID.
• This caused a "You must choose a space." error in the UI.

Instead, populate the control with the effective value instead of the literal database value. This makes the edit go through cleanly.

Also add a note about this for future-me.

Test Plan:

• Disabled "Visibility" control in task edit form.
• Edited an old task which had null as a spacePHID in the database.
• Before patch: UI error about selecting a Space.
• After patch: edit goes through cleanly.

Reviewers: chad, 20after4

Reviewed By: chad, 20after4

Subscribers: 20after4, aklapper

Differential Revision: https://secure.phabricator.com/D15306